Use one of the two predefined instances index_exp or s_
rather than directly using IndexExpression.

For any index combination, including slicing and axis insertion,
a[indices] is the same as a[np.index_exp[indices]] for any
array a. However, np.index_exp[indices] can be used anywhere
in Python code and returns a tuple of slice objects that can be
used in the construction of complex index expressions.